Dimensionally recoverable articles such as heat-recoverable tubing are well known. Such articles are often used to provide electrical and environmental insulation over spliced wires or to protect areas of cables where there are breaks in the insulation. To provide adequate recoverability and flexibility, the articles generally comprise polymers. For many applications it is necessary that the polymer be flame-retardant in order to minimize the risk of damage in the event of a fire. Such flame-retardancy may be achieved either by the use of a halogen-containing polymer, for example a fluoropolymer, or by the addition of a halogenated material, for example a brominated or chlorinated compound such as decabromodiphenylethane, to a non-halogenated polymer, for example polyolefins such as polyethylene, ethylene copolymers or polyesters. An example of conventional halogenated tubing is DR25™ tubing, available from TE Connectivity.
Halogen-containing polymeric compounds with such flame retardancy are useful as they self-extinguish when removed from flame. However, they do have disadvantages, as they can undergo continuous combustion to produce a large quantity of smoke, or they can thermally decompose, releasing corrosive gases such as halogen acids which can be detrimental to personnel or sensitive electronic equipment.
Attempts have been made to incorporate halogen-free flame-retardants into polymer resins to overcome the above disadvantages. Halogen-free flame-retardants such as alumina trihydrate (ATH) must be used in relatively large concentration in order to give fully useful flame-retardancy to the resulting compositions. However, when a large concentration of halogen-free flame-retardant is added to a polymer resin, the resulting resin composition, when extruded, exhibits reduced physical properties, especially in tensile strength and elongation, and also greatly reduced aging and electrical properties. In addition, these compositions are difficult to extrude and expand into tubing, especially thin wall tubing.
When a composition is used in the form of tubing, a number of product-related flame tests are conducted. Of particular importance for Military and Aerospace applications are IEC 60684-3-271, VG 95343-5 Type D, SAE-AS-I-23053/16 which contains several different aspects, including measurements of the physical properties such as tensile strength and elongation, and a measurement of the performance of the tubing when exposed to a flame. According to VG 95343-5 Type D, combustion behavior test number 5.12.1.1, a section of heat shrink tube of size approximately 150 mm in length is shrunk on to a metal mandrel of approximately 200 mm in length having a diameter 10% greater than the inner diameter of the freely shrunk tubing. The test specimens are then marked and placed in the combustion chamber, marking the position of the specimen, point of application of the flame etc. according to the prescribed test method. For a flame applied for 60 seconds, to pass the flame test, the combustion must extinguish of its own within 15 seconds after the flame has been removed for type D products and 30 seconds for Type L products. It is also desirable that the tubing have adequate performance for continuous use at elevated temperature, for example at 150° C. for 3000 hours. Such high temperature performance is particularly important when the tubing is used for military applications.
While halogen-containing tubing has acceptable physical and flammability properties, for some markets, a zero-halogen containing material is required. As described above, however, known halogen-free polymers have poor physical properties.